This invention relates to high intensity discharge lamps and more particularly to a dose for electrodeless high intensity discharge devices containing sodium halide and scandium halide in predetermined proportions to enhance the device efficacy and to attain longer lamp life.
Since the invention of the high pressure mercury lamp, there has been a large effort to maximize the visible radiation from the plasmas in high intensity discharge lamps. Two of the major problems encountered in this effort have been the production of radiation outside the visible part of the spectrum and the loss of energy from the arc tube itself as infrared radiation because the arc tube must be hot enough to vaporize radiating atoms or it becomes heated unavoidably by the hot plasma. With high pressure sodium lamps, there is practically no energy wasted by radiative output in the ultraviolet, but the plasma still radiates significant amounts of power in the infrared, and nearly half of the input energy is lost as infrared incandescence of the very hot alumina arc tube.
A large variety of so-called mercury-metal halide arc discharge lamps have been investigated. British Patent Specification No. 1,444,023 discloses high pressure electrical discharges in a gas atmosphere containing aluminum halide, sodium chloride or iodide, mecury and a rare gas. U.S. Pat. No. 4,422,011 discloses high-pressure mercury vapor discharge lamps containing a rare gas, mercury, sodium halide such as sodium iodide and at least one halide of rare earth metals Ce, Pr, Nd and Lu. Sodium iodide in these lamps is intended to provide sodium atoms in the plasma as one of the principal radiators. In order to attain sodium iodide vapor pressures of several torr required to produce sodium radiation in the arc, the minimum arc tube wall temperature must be 750.degree. C., and this means that nearly one-half of the input power in these lamps is again lost as infrared incandescence of the arc tube.
Scandium iodide is often added to sodium iodide in these arcs to enhance the color rendering qualities and efficacy. Sodium and scandium halides are known to form complexes NaScX.sub.4 which have higher vapor pressure than the corresponding sodium halide NaX alone. C. Hirayama, et al. (Journal of IES, page 209, July, 1977), studying in particular the complex compound NaI-ScI.sub.3, discussed that in commercial lamps where the NaI/ScI.sub.3 ratio ranges from 15-30, the scandium vaporizes predominantly as the NaScI.sub.4 complex and that since this ratio is high, there also is a significant contribution to the vapor from the sodium iodide. Although at low NaI/ScI.sub.3 ratios the vaporization of both the sodium and scandium is dominated by the complex with the consequence that the vapor concentration of both radiators is greatly enhanced, the sodium vapor pressure enhancement is negligible in the typical range of the NaI/ScI.sub.3 ratio in commercial lamps, and wall temperature must still be maintained relatively high.
In U.S. Pat. No. 3,979,624, bromides as well as iodides and mixtures of bromides and iodides were considered. In the presence of electrodes made of a chemically reactive material such as tungston, however, iodides are more frequently used.